Description: This version of dfsb72146 does not require that and be
distinct. This permits it to be used as a definition for substitution
in a formalization that omits the logically redundant axiom ax-171623 i.e.
that doesn't have the concept of a variable not occurring in a wff.
(df-sb1656 is also suitable, but its mixing of free and
bound variables
is distasteful to some logicians.) (Contributed by NM, 26-Jul-2006.)
(Proof shortened by Andrew Salmon,
25-May-2011.)